On Mon, Jan 2, 2023 at 11:50 AM Roger Sayle <ro...@nextmovesoftware.com> wrote:
>
>
> This patch modifies the way that ix86_expand_int_movcc generates RTL,
> to allow the condition mask to be shared/reused between multiple
> conditional move sequences. Such redundancy is common when RTL
> if-conversion transforms non-trivial basic blocks.
>
> As a motivating example, consider the new test case:
>
> int a, b, c, d;
> int foo(int x)
> {
> if (x == 0) {
> a = 3;
> b = 1;
> c = 4;
> d = 1;
> } else {
> a = 5;
> b = 9;
> c = 2;
> d = 7;
> }
> return x;
> }
>
> This is currently compiled, with -O2, to:
>
> foo: cmpl $1, %edi
> movl %edi, %eax
> sbbl %edi, %edi
> andl $-2, %edi
> addl $5, %edi
> cmpl $1, %eax
> sbbl %esi, %esi
> movl %edi, a(%rip)
> andl $-8, %esi
> addl $9, %esi
> cmpl $1, %eax
> sbbl %ecx, %ecx
> movl %esi, b(%rip)
> andl $2, %ecx
> addl $2, %ecx
> cmpl $1, %eax
> sbbl %edx, %edx
> movl %ecx, c(%rip)
> andl $-6, %edx
> addl $7, %edx
> movl %edx, d(%rip)
> ret
>
> Notice that the if-then-else blocks have been if-converted into four
> conditional move sequences/assignments, each consisting of cmpl, sbbl,
> andl and addl. However, as the conditions are the same, the cmpl and
> sbbl instructions used to generate the mask could be shared by CSE.
>
> This patch enables that, so we now generate:
>
> foo: cmpl $1, %edi
> movl %edi, %eax
> sbbl %edx, %edx
> movl %edx, %edi
> movl %edx, %esi
> movl %edx, %ecx
> andl $-6, %edx
> andl $-2, %edi
> andl $-8, %esi
> andl $2, %ecx
> addl $7, %edx
> addl $5, %edi
> addl $9, %esi
> addl $2, %ecx
> movl %edx, d(%rip)
> movl %edi, a(%rip)
> movl %esi, b(%rip)
> movl %ecx, c(%rip)
> ret
>
> Notice, the code now contains only a single cmpl and a single sbbl,
> with their result being shared (via movl).
>
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check, both with and without --target_board=unix{-m32},
> with no new failures. Ok for mainline?
>
>
> 2023-01-02 Roger Sayle <ro...@nextmovesoftware.com>
>
> gcc/ChangeLog
> * config/i386/i386-expand.cc (ix86_expand_int_movcc): Rewrite
> RTL expansion to allow condition (mask) to be shared/reused,
> by avoiding overwriting pseudos and adding REG_EQUAL notes.
>
> gcc/testsuite/ChangeLog
> * gcc.target/i386/cmov10.c: New test case.
OK.
Thanks,
Uros.
>
>
> Thanks in advance,
> Roger
> --
>
